Thermal responsive automatic switch



THERMAL RESPONSIVE AUTOMATIC SWITCH Filed NOV. 26, 1949 2 Sheets-Sheet l INVENTOR.

A T T NNy 5 K. MIZUTANi 2,647,971

THERMAL RESPONSIVE AUTOMATIC SWITCH Filed Nov. 26, 1949 2 Sheets-Sheet 2 I l. I I

IN V IL'NTOR. k 4

Patented Aug. 4, 1953 THERMAL RESPON SIV E AUTOMATIC SWITCH Kinichi Mizutani, Yokohama, Japan Application November 26, 1949, Serial No. 129,569

3 Claims. 1

My invention relates to improvements in thermal responsive automatic switches of quickoperating type and more particularly of no-hinge floating switch lever type, and has for its principal object to provide an automatic and quickly operating electric switch which normally maintains positive contact and safely cuts off a comparatively large current at a predetermined temperature rise with smaller size and is positive in operation and can stand for repeated offand on operations without damage.

Another object of this invention is to provide a safety switch for preventing the burning of pole transformers in case of over-load.

A further object of this invention is to provide a rolling and wiping contact type switch which maintains normally positive contact and prevents spark and over-heating of the contact members.

A further object of this invention is to provide a quick-operating switch which has no hinged switch levers, thereby simplifying the construction and obviating troubles due to complicated parts as in heretofore usual switches of this kind.

With the above and other objects in view the invention consists of certain novel features of a plurality of bi-metals and also combination of floating switch lever and spring hereinafter fully described and claimed, it being understood that various modifications and applications of the switch may be resorted to within the scope of the appended claims without departing from the spirit of the invention.

In the accompanying drawing forming a part of this specification:

Fig. 1 is a side elevation partially in section of an automatic snap switch embodying this invention;

Fig. 2 is a plan view of Fig. 1 showing the switch lever at a closed contact condition;

Fig. 3 is a similar view to Fig. 2 showing the contact open condition of the switch lever;

Fig. 4 is a side elevation of thermal responsive elements adopted in the switch of this invention;

Fig. 5 is a sectional plan view of the thermal responsive elements shown in Fig. 4;

Fig. G-a and b are elevation and side view of the curved plate spring;

Fig. 7 is a connection diagram of the switch when it is applied to the over-load protection of a pole transformer.

Defects in heretofore usual thermal switches of a bi-metal type are due to the use of a straight bi-metal which bends at a rise of temperature and also in that careful investigations Were not Japan January 26, 1947 paid for construction and arrangement of working parts so that the thermal elements cannot develop suflicicntly large operating force and the circuit interrupting capacity is not only small but also the contacts are liable to burn away owing to the slow cut-off at a temperature change.

This invention is to obviate the above defects and provide a thermal responsive quick operating automatic switch which is characterized in that a plurality of bi-metal strips bent to a suitable curved shape such as circular arc spiral and the like curve so as to have a large elasticity by its own configuration are arranged in such manner that the free or operating ends of the co-operating two bi-metals fixed at the other end deflect in opposite directions with each other at a temperature change and the switch lever is held under floating condition at its middle point by the free end of one of the bi-metal and the other end of the switch lever engages with one end of a spring lever which engages at its other end with the free end of the other bi-metal so that the spring lever is deformed to store energy by the deflection in opposite directions of the co-operating free ends of the bi-metals and then operates to quickly turn the switch lever, thereby opening or closing the contact.

Now referring to Figs. 1 to 6 which illustrate an embodiment of my invention as applied to an over-load switch of pole transformers; l and 2 designate a couple of thermal responsive elements; 3 is an elastic lever such as a fiat spring strip; 4 represents a switch lever; 5 represents fixed support or frame; 6 is a fixed contact; 1 is a movable contact; and 8 is a base-plate. The thermal responsive elements I and 2 consists of bi-metals of elastic strip bent to an arcuate shape and are fixed to the stationary supporting member 5 at one end, while their free ends being extended in the same direction along almost concentric circular arcs. The spring lever 3 consists of a arcuate fiat spring strip having projections I3 and H4 at its end as shown in Fig. 6 and arranged to pass through a slot l0 provided for the oi-metal 2. One end projection l3 fits in a slot 9 provided for the bi-metal l at the free end thereof, while the other projection l4 engages in a slot l2 provided at one end of theswitch lever A to support the latter. The lower part of the switch lever 4 passes through a slot ll provided near about the free end of the bi-metal 2 and supported thereby. I5 represents a flexible conductor; l6 and "represent terminal set screws, and I8 represents a cover.

The thermal responsive elements I and 2 used in this invention can be made by laminating and welding together two metal plates having different coeflicients of thermal expansion and drawn to a required thickness and out to a strip of suitable breadth. Moreover the elements I and 2 are so constructed that they elongate or contract in opposite directions with each other corresponding to the change of temperature as shown by the arrows in Figs. 1 and 2.

The operation of the above described thermal responsive automatic quick break switch of this invention is explained in the following. If the temperature of the bi-metals or the surrounding medium of the switch is below avpredetermined degree, that is, under the normal condition, the free end of the thermal responsive element l retires to the contracted position while the free end of the element 2 occupies the elongated .position as shown in Fig. 1 so that the spring lever 3 is held at its middle portion by the rear edge of the slot 59 of the element 2 and forces the switch lever by the rear end of the spring lever 3 since the top end of the lever being pulled downwardly by the engagement with the free end of the bimetal I by means of the projection l3. Under such condition, the switch lever 1 tends to turn counter-clockwise about the hinge support on the rear edge of the slot ll of the bi-met'al 2 so that the contact member '1 attached to the free end of the switch lever A is pressed on the stationary contact piece 6, thereby maintains good electrical contact.

Next if the ambient temperature of the switch becomes higher corresponding to the load condition, the bi-metals l and 2 tend to deflect in the directions shown by the full line arrows in Figs. 1 and 2, but the slow deflections are prevented by the spring force of the lever 3 which is balanced by the operative forces of the bi-metals l and 2 and holds the switch lever 4 in the condition as shown in Fig. 1 by giving almost constant pressure to the contact members. When the temperature becomes higher and the counteracting iorces of the bi-metals i and 2 predominate the force of the spring lever 3 the circuit openin operation is commenced, but during the initial movement the operating parts are only shifted to the position for automatically opening the switch by the difference of forces acting towards the circuit opening position. Accordingly after the initial movement towards the contact opening position the spring lever 3 is forced to bend sufiiciently until apredetermined temperature is reached and stores sufficient energy for operating the switch lever to the open position and maintains suflicient contact pressure. Such function is mainly effected by the operating force caused by the deflection of the bi-metal l. Finally when the predetermined temperature is arrived the operative force stored in the bi-metal 2 predominates and overcomes the elasticity of the spring lever 3 so that the total stored energy is discharged suddenly and the bi-metal 2 begins to move quickly in the direction shown by the full line arrow in Fig. 1. Thus the motion of the total switch mechanism is accelerated to shift the rear part of the switch lever t quickly to the upward of Fig. l. The upper projection [3 of the spring lever 3 engaging the free end of the bi-metal l which was expecting to move towards the switch opening position at this moment will quickly pass over the centre line a-a of the switch mechanism and at the same instant the lower end of switch lever A is pulled up instantaneously by the rear end of the spring 3 so that cuit. The operation is reverse to that above described. In the open position of the switch shown in Fig.3;the projected tip of the bi-"netal l is made to engage the crooked portion of the floating switch lever t to support it mechanically and the open position of the lever is designed to come near about the centre line of the switch mechanism and the distance from the beginning of closing operation of each part of the switch mechanism to the position of passing over the centreline a-a' in Fig. 2 can be selected much shorter than that in case of opening the switch. With such arrangement, when the temperature descends to apredetermined degree, the bi-metals I and 2 commence to deflect in the direction shown by the dotted line arrows in 2 and all parts of the switch mechanism start almost simultaneously the closing movement towards the switch closing direction so that the upper projection l3 of the operating spring lever 3 passes quickly over the centre line a'-c. At this moment the spring lever 3 shifts the switch lever 4 to the closing position by instantaneous operation so that the moving contact member 7 makes impulsive contact with the stationary contact 6 to close the circuit and all parts occupy the position shown in Fig. 1.

When the contact is opened the free end of the bi-metal 2 supporting the switch lever 4 is slightly shifted back by the force of spring lever 3 while the free end moves from the closed to open position of the contact and accordingly the contact member 1 makes the wiping action against the contact 5. This wiping action is effective in polishing the contact surface to keep it clean and also the position at which are or spark occurs when the movable contact is departed from the stationary contact is made different from the current flowing contact surface while the contact is closed normally. By the impulsive contact in closing the contacts, dusts and oil films on the contact surface can be removed and the rough surface caused by are or spark is recovered and the wiping contact action occurs in the direction opposite to that of switch opening operation. The above action is effective to prevent the increase of contact resistance against the current flow and avoid the chance of annealing, sticking and burning of contact pieces, thereby increasing durability.

Referring to Fig. 7 which shows a connection diagram of the present switch as applied to a primary cut-out switch for over-load protection of a transformer, l9 represents power lines; 20 and 2! represent primary and secondary windings respectively of the transformer; 22 represents feeder lines; 23 and 24 are taps of the primary Winding. A suitable tap 23 is connected through a wire it with the stationary contact 6 of the present switch and the other tap 24 leads through the wire H to the fixed support 5 which is electrically connected with the switch lever 4 by a flexible cord i5, thereby completing a primary circuit.

The present switch is dipped into oil in the transformer casing and if over-current flows for a certain duration and the temperature of oil becomes higher than a predetermined degree the switch of this invention operates to open the contact 1 and cuts off the circuit, thereby preventing burning of transformer. When the oil temperature descends to a predetermined degree the present switch operates to reclose the circuit. Thus a circuit of comparatively large current capacity can be cut-off safely and positively. According to the result of actual tests the switch of this invention immersed in oil operated at the temperature difference of only a few degrees C. and electric current of several ten amperes at 3300 volts was directly cut oil positively and the switch stood for repeated cut-off and put-in tests of more than several ten thousand times.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermal responsive automatic switch which comprises a base, a plurality of co-operating thermal responsive elements bent to approximately circular form and arranged approximately concentrically by being fixed at one end thereof to the base, a switch lever having a floating fulcrum, a switch contact at one end of the switch lever, a curved spring member engaging at one end with the free end of one of said thermal responsive elements and at the other end engaging with the end of the switch lever that is opposite from the contact-carrying end, said switch lever being engaged and actuated by the free end of another one of said thermal responsive elements, the thermal responsive elements switch lever and spring member being so arranged that when the temperature of said thermal responsive elements becomes higher than a predetermined degree, the force caused by the relative deformation of said thermal responsive elements acts to bend said spring member for quickly turning said switch lever about its floating fulcrum by the stored spring energy, thereby opening the switch contact by snap action,

2. A thermal responsive automatic switch which comprises a base, a plurality of co-operating thermal responsive elements bent to approximately circular form to have elasticity by their own configuration and arranged approximately concentrically, said elements being fixed at one end thereof to the base, so as to elongate or contract in opposite directions to one another with temperature change, a floatin switch lever provided with a movable contact member at the free end thereof and supported at the middle part by the free end of one or said thermal responsive element, a curved spring lever engaging at one end with the free end of said switch lever opposite from the contact carrying end, and engaging at the other end with the free end of one of said thermal responsive elements, said thermally responsive elements, switch lever and spring lever being so arranged that when said thermal responsive elements deflect in opposite directions said spring lever is biased to bend, thereby stor-,

ing the spring energy to turn said switch lever quickly about the floating fulcrum when the cooperating ends of said thermal responsive elements pass over the dead centre line of said switch lever.

3. A thermal responsive automatic quick operating switch which comprises a pair of elastic bimetal strips bent to approximately circular form and arranged approximately concentrically by being fixed at one end with free ends extended towards the same direction, a switch lever having a movable contact at the free end thereof and being supported in floating condition at its middle portion only by the operating end of one of said bi-metal strips, a curved spring lever engaging at one end with the free end of said switch lever opposite from the contact carrying end, and at the other end with the free end of the outer bi-metal and also at the middle part with the inner bi-metal through a slot thereof, said strips, switch lever and spring lever being so associated and arranged that said switch lever is quickly turned about a floating fulcrum for a certain angle by the stored energy of said spring lever when the free ends of co-operating bi-metals deflect upon a predetermined change of temperature.

KINICI-II MIZU'I'ANI.

References Cited in the file of this patent UNITED STATES PATENTS 

